1. Field of the Invention
This invention relates to spacer fluids which are useful in displacing a first drilling fluid from a wellbore space with a second incompatible fluid.
2. Description of the Prior Art
In rotary drilling of wells a drilling fluid, sometimes referred to as a drilling mud, is circulated downwardly through a pipe, sometimes referred to as a drill string, and back up the annulus between the drill string, and the inside of the wellbore. Drilling fluid performs various functions such as cooling the drill bit, maintaining the desired pressure in the well, carrying drill cuttings upwardly through the annulus between the drill string and the wellbore face and the like as well known to those skilled in the art. The drilling fluid can contain many different chemicals but will most often contain a viscosifier such as bentonite. When a casing or liner is to be cemented into the wellbore, any drilling fluid and remnants of the viscosifier present in the wellbore are preferably removed to aid the bonding of the cement between the casing or liner and the wellbore. In removing this drilling fluid from the wellbore and to clean the annulus, a wash or spacer fluid can be introduced ahead of a cement slurry.
Spacer fluids are conventionally used in cementing operations relating to well completion as follows. Drilling fluids and cement slurries are typically chemically incompatible fluids which undergo severe gelation or flocculation if allowed to come into contact. Thus the drilling fluid must be removed from the wellbore annulus immediately prior to cement placement in the annulus. Spacer fluids are pumped between the drilling fluid and the cement slurry to form a buffer between the drilling fluid and the cement slurry, clean the annulus and prevent the drilling fluid and the cement slurry from coming into contact.
Spacer fluids should possess certain rheological tendencies which assist in granular solids removal and which encourage removal of the drilling fluid filter cake from the walls of the well. A common cause of failure in primary cementing is the incomplete displacement of drilling fluids which results in the development of mud filled channels in the cement. These mud filled channels may open during well production permitting vertical migration of oil and gas behind the casing.
Conventional spacer fluids are typically composed of an aqueous base fluid and a weighting agent. The weighting agent is included in the composition to increase the density of the spacer fluid to a desired value and to increase the erosion effect of the spacer fluid on the filter cake clinging to the walls of the well.
The fundamental properties of the aqueous base spacer fluid are typically particle stability and suspension (anti-settling properties), fluid-loss control, favorable rheology, and compatibility with drilling fluids and cement slurries. These properties are directly related to the composition of the spacer fluid.
Consequently a conventional spacer base fluid may include one or more of an anti-settling agent, a fluid-loss controlling agent, a dispersing agent, and a surfactant for obtaining a water wetted surface to aid in cement bonding. The final composition of conventional spacer fluids is typically obtained by adding a weighting agent to the aqueous base spacer fluid to achieve a desired fluid density.
The anti-settling agent and fluid-loss controlling agent may comprise a single component of the composition or may comprise a plurality of components of the composition. The component agents typically are soluble or dispersible in water. Dependent upon the water available at the site and in the geological strata encountered in the wellbore, the aqueous base spacer fluid typically includes fresh water, sea water, brine or an aqueous composition containing one or more dissolved salts such as sodium chloride, potassium chloride, magnesium chloride and calcium chloride. It is preferred that the spacer fluid retain its above mentioned fundamental properties at all possible salt concentrations. Spacer fluids are conventionally used over a wide temperature range from the surface temperature to the bottom hole circulating temperature in a wellbore. The bottom hole circulating temperature may be 200.degree. C. or higher in the case of certain geothermal wells. The term "anti-settling properties" refers to the capacity of the spacer fluid to keep the weighting agent particles in stable suspension throughout the cementing operation which may typically last from about 1 to about 4 hours or longer. A spacer fluid is considered to have good fluid loss control properties if the fluid loss measured according to API specification 10, Appendix F is less than 100 milliliters/30 minutes and excellent if the fluid loss is less than 50 milliliters/30 minutes. Favorable rheology for a spacer fluid requires that the fluid has minimum friction pressure while maintaining adequate suspension of solids. Since the spacer fluid is to be pumped between the drilling fluid and the cement slurry for removing and replacing the drilling fluid in the well annulus, it is very important the spacer fluid be as compatible as possible with both the drilling fluid and the cement slurry.
The compatibility of a spacer fluid with a drilling fluid and a cement slurry is determined in the laboratory by studying the viscosity of binary or ternary mixtures of spacer fluid with the drilling fluid and/or cement slurry varying over the range of 0 to 100 percent by volume for each component of the mixture. Such compatibility in the past has been difficult to obtain primarily because the drilling fluid and the cement slurry are incompatible fluids.
The compatibility of the spacer fluid with the drilling fluid and the cement slurry is considered to be excellent if the viscosity of a mixture of the spacer fluid and the drilling fluid or the cement slurry at a given shear rate and temperature is equal to or less than the viscosity of the more viscous component of the mixture at the same shear rate and temperature. Likewise, the viscosity of a mixture of all three components is considered to excellent if it is less than or equal to the viscosity of the most viscous component at the same shear rate and temperature.
Conventional spacer fluid compositions do not usually demonstrate good compatibility with mixtures of drilling fluids and cement slurries while simultaneously possessing good rheological fluid loss control and anti-settling properties over the entire range of shear rates and temperatures normally encountered in oil field services.
Spacer fluids using sulfonated styrene-maleic anhydride copolymer (SSMA) have previously been used. Such spacer fluids are disclosed in U.S. Pat. No. 5,030,366 "Spacer Fluids" issued Jul. 9, 1991 to Wilson et al; U.S. Pat. No. 5,113,943 "Spacer Fluids" issued May 19, 1992 to Wilson et al; and U.S. Pat. No. 5,292,367 "Dispersant Compositions for Subterranean Well Drilling and Completion", issued Mar. 8, 1994 to Bloys et al. These patents disclose spacer fluids containing dispersing materials which have many of the desired properties of spacer fluids. These patents are hereby incorporated in their entirety by reference.
The preferred dispersants disclosed in these patents are sulfonated styrene-maleic anhydride copolymer (SSMA) and sulfonated vinyl toluene-maleic anhydride copolymer (SVT-MA).
Unfortunately, SSMA and SVT-MA have not always been commercially available in sufficient quantities for oil field operations. As a result, it has been difficult to obtain sufficient quantities of SSMA and SVT-MA to satisfy the demand for dispersants in spacer fluid compositions. Accordingly, a continuing search has been directed to the discovery of additional dispersant materials which can be used to produce effective spacer fluids. As previously noted drilling fluids and cement slurries are incompatible fluids and few dispersant materials are effective to disperse mixtures of the two.